收集数据

A website asks visitors "Do you like this website?" and 92% click Yes. A survey at a gym finds that most people exercise daily. Both numbers are real, and both are nearly useless, because of who was asked. This unit is about the decisions made before any data exist, which decide what the data can ever tell you.

A conclusion is only as good as the data behind it. How data are collected decides what you may conclude – whether you can generalize to a population 总体, and whether you can claim cause and effect. Poorly collected data can be worse than none.

• In an observational study 观察性研究 you measure individuals without trying to influence them. It can show association, but not causation, because lurking variables may explain the link.
• In an experiment 实验 you deliberately impose a treatment 处理 and compare responses. A well-designed experiment can establish cause and effect.

To learn about a population you take a sample 样本. Random sampling 随机抽样 protects against selection bias 偏差 and lets you generalize (it cannot fix undercoverage, nonresponse, or response bias — see below). Common designs:

• Simple random sample (SRS) 简单随机样本: every group of the chosen size is equally likely.
• Stratified 分层: split the population into similar strata, then sample within each.
• Cluster 整群: split into clusters, randomly choose whole clusters.
• Systematic 系统: pick every $k$th individual from a random start.

A convenience sample 方便样本 or voluntary response sample is not random and is biased.

Worked example. To survey a school, an administrator lists all students by grade and randomly selects $20$ from each grade. This is a stratified sample – the grades are the strata – which guarantees every grade is represented, unlike an SRS that might by chance draw few from one grade.

Common mistakes. Mixing up stratified and cluster samples. Stratified: split into groups of similar individuals and take a random sample from every group. Cluster: split into groups that each look like the whole population, randomly choose some groups, and survey everyone in them. Also, calling a sample random when it is only convenient (“the first $50$ students in the canteen”).

Bias makes estimates systematically miss the truth:

• Undercoverage 覆盖不足: some groups are left out of the sampling frame.
• Nonresponse 无回应: selected people do not answer.
• Response bias 回应偏差: people answer inaccurately (bad wording, sensitive topics).

Bias is about a consistent error in one direction – increasing the sample size does not fix it.

Common mistakes. Saying a larger sample reduces bias: it reduces variability only. Naming a bias without saying, in context, which way it pushes the estimate (too high or too low) and why.

Good experiments follow three principles:

• Comparison with a control group 对照组 (often a placebo 安慰剂).
• Random assignment 随机分配 of subjects to treatments, to balance out other variables.
• Replication 重复: enough subjects per treatment to see a real effect.

Confounding 混杂 occurs when another variable is tied to the treatment so their effects cannot be separated; random assignment guards against it. Blinding 盲法 hides who is getting which treatment to prevent expectation effects: in a single-blind 单盲 study only one side is kept unaware (usually the subjects, or only the people assessing the result), while in a double-blind 双盲 study neither the subjects nor the researchers who interact with them know, blocking both the placebo effect and biased assessment. Blocking 区组 groups similar subjects and randomizes within each block to reduce variability.

Worked example: describing random assignment. Forty volunteers will test two study apps, A and B, 20 per app. A full-credit description names the method precisely enough that someone else could carry it out:

Number the volunteers 1 to 40. Use a random number generator to choose integers from 1 to 40, ignoring repeats, until 20 different numbers are chosen. Those 20 volunteers use app A; the remaining 20 use app B.

"Randomly put them into two groups" earns nothing: it does not say how. Drawing names from a hat also works, if you say the slips are the same size and well mixed.

Match the design to the goal: use a completely randomized design for uniform subjects; a randomized block design when a known variable (sex, age) affects the response; a matched-pairs design when each subject can serve as its own control. State how you would carry out the randomization.

Worked example: blocking. The study-app volunteers are 24 Grade 10 and 16 Grade 11 students, and grade level surely affects test scores. A randomized block design 随机区组设计 separates the two grades first. Within each grade, randomly assign half to app A and half to app B: 12 and 12 in Grade 10, 8 and 8 in Grade 11. Then compare the apps within each grade. Grade differences can no longer hide, or exaggerate, the effect of the app.

Two questions decide the scope of a conclusion:

• Random assignment used? Then a significant difference can be attributed to the treatment (causation) – for these subjects.
• Random sampling from a population? Then results generalize to that population.

Only an experiment with random assignment supports a cause-and-effect claim; only random sampling supports generalization. Say exactly which you have.

Worked example. Researchers randomly assign $100$ volunteers to a new drug or a placebo, and the drug group improves significantly more. Because of the random assignment, the improvement can be attributed to the drug (causation) – but because the subjects were not randomly sampled, the conclusion applies only to these volunteers and does not automatically generalize to everyone.

Read the design once for each question: was the sample chosen at random (generalize?), and were the treatments assigned at random (causation?).

Common mistakes. Claiming cause and effect from an observational study; generalizing an experiment on volunteers to a whole population; and confusing random sampling (who is in the study) with random assignment (who gets which treatment).

Answer each before reading the answers below.

1. A teacher surveys the first 30 students to arrive at school. Name the sampling method and one likely bias.
2. A school has 900 students in 30 homerooms. The principal randomly selects 3 homerooms and surveys every student in them. Name the design.
3. Describe how to randomly assign 30 tomato plants to two fertilisers, 15 each.
4. A study finds that people who drink more coffee live longer, from medical records. Can it conclude that coffee causes longer life? Explain.
5. A student says: "A sample of 10,000 people from a voluntary online poll is better than a random sample of 1,000." Correct the student.

1. A convenience sample. Early arrivers may differ systematically, for example by living closer or by being more organised, so the results may be biased.
2. A cluster sample: the homerooms are clusters, and every member of each chosen cluster is surveyed.
3. Number the plants 1 to 30. Use a random number generator to choose 15 different numbers from 1 to 30; those plants get fertiliser A and the other 15 get fertiliser B.
4. No. This is an observational study. Coffee drinkers may differ in other ways, such as income or health, which could explain the longer lives. Only random assignment of coffee could support causation.
5. A voluntary response sample is biased: people with strong opinions are more likely to respond. A larger biased sample is still biased; it only makes the wrong answer look more precise. The random sample of 1,000 is better.

• Distinguish an observational study (finds association) from an experiment (can show causation).
• Good sampling is random (SRS, stratified, cluster) — beware bias (voluntary response, undercoverage, nonresponse).
• Good experiments use control, randomization, and replication; blocking handles a known nuisance variable.
• Only a randomized experiment supports a cause-and-effect conclusion.
• Name the population, sample, and any confounding clearly.